Hmm, just taking a look over, I can see nothing immediately wrong. Although opinions are like bumholes, everyone has them.
In the provided model, the rail network that transports everything to, and inside the gun apear to lead underground. It takes a lot of work to dig underground train tunnels, even from scratch without worrying about city plumbing and pipes.
You might want to say that the rail network is seperate from the rails leading into the gun, possibly mention a marshalling yard, and put in a bit of fluff about the rail network combining the planets current infrastructure, with NMX additions.
Oh, and uh, for construction. Perhaps saying that the structure is made prefab, so it can be constructed quickly, and that to make the hole to bury the base of the gun in they simply drop a large cratering bomb. Simple, fast, cheap.
Anyway, apart from that I still reckon it should be called The Marinator.